netdev - Re: [PATCH net] packet: call fanout_release, while UNREGISTERING a netdev

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Message-ID: <1485805493.6360.113.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Mon, 30 Jan 2017 11:44:53 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Anoob Soman <anoob.soman@...rix.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net] packet: call fanout_release, while UNREGISTERING a
 netdev

On Mon, 2017-01-30 at 19:08 +0000, Anoob Soman wrote:
> 
> On 30/01/17 17:26, Eric Dumazet wrote:
> > On Thu, 2016-10-06 at 20:50 -0400, David Miller wrote:
> >> From: Anoob Soman <anoob.soman@...rix.com>
> >> Date: Wed, 5 Oct 2016 15:12:54 +0100
> >>
> >>> If a socket has FANOUT sockopt set, a new proto_hook is registered
> >>> as part of fanout_add(). When processing a NETDEV_UNREGISTER event in
> >>> af_packet, __fanout_unlink is called for all sockets, but prot_hook which was
> >>> registered as part of fanout_add is not removed. Call fanout_release, on a
> >>> NETDEV_UNREGISTER, which removes prot_hook and removes fanout from the
> >>> fanout_list.
> >>>
> >>> This fixes BUG_ON(!list_empty(&dev->ptype_specific)) in netdev_run_todo()
> >>>
> >>> Signed-off-by: Anoob Soman <anoob.soman@...rix.com>
> >> Applied and queued up for -stable, thanks.
> > This commit (6664498280cf "packet: call fanout_release, while
> > UNREGISTERING a netdev")
> > looks buggy :
> >
> > We end up calling fanout_release() while holding a spinlock
> > ( spin_lock(&po->bind_lock); )
> >
> > But fanout_release() grabs a mutex ( mutex_lock(&fanout_mutex) ), and
> > this is absolutely not valid while holding a spinlock.
> 
> Yes, that is wrong.
> 
> >
> > Anoob, can you cook a fix, I guess you have a way to reproduce the thing
> > that wanted a kernel patch ?
> >
> > (Please build your test kernel with CONFIG_LOCKDEP=y)
> 
> Sure, I am planning to move fanout_release(sk) after 
> spin_unlock(bind_lock). Something like this.
>                                  }
>                                  if (msg == NETDEV_UNREGISTER) {
>                                          packet_cached_dev_reset(po);
> -                                       fanout_release(sk);
>                                          po->ifindex = -1;
>                                          if (po->prot_hook.dev)
> dev_put(po->prot_hook.dev);
>                                          po->prot_hook.dev = NULL;
>                                  }
>                                  spin_unlock(&po->bind_lock);
> +                               if (msg == NETDEV_UNREGISTER) {
> +                                       fanout_release(sk);
> +                               }
>                          }
>                          break;
> 
> I will quickly test it out.

It wont be enough.

You need to also fix a race if two cpus call fanout_release(sk) at the
same time.

Thanks.